Implemented checkRequired for SchemaType Mixed

[Reggino]

See issue #1718

[ebensing]

Benchmarks for c565729:
insert

Insert - Mongoose - Basic x 276 ops/sec ±65.42% (74 runs sampled)
Insert - Driver - Basic x 565 ops/sec ±7.25% (84 runs sampled)
Insert - Mongoose - Embedded Docs x 94.36 ops/sec ±8.34% (50 runs sampled)
Insert - Driver - Embedded Docs x 501 ops/sec ±4.77% (83 runs sampled)

delete

Delete - Mongoose x 578 ops/sec ±1.90% (84 runs sampled)
Delete - Driver x 614 ops/sec ±1.77% (85 runs sampled)

update

Update - Mongoose - Basic x 517 ops/sec ±3.49% (86 runs sampled)
Update - Driver - Basic x 593 ops/sec ±1.04% (88 runs sampled)
Update - Mongoose - Embedded Docs x 88.55 ops/sec ±8.46% (50 runs sampled)
Update - Driver - Embdedded Docs x 439 ops/sec ±1.04% (88 runs sampled)
Update - Mongoose - Multiple Documents x 304 ops/sec ±7.24% (77 runs sampled)
Update - Driver - Multiple Documents x 491 ops/sec ±3.43% (82 runs sampled)
Update - Mongoose - pop and push x 29.96 ops/sec ±10.23% (54 runs sampled)
Update - Mongoose - Array Manipulation, parallel ops x 228 ops/sec ±6.97% (65 runs sampled)
Update - Mongoose - findOneAndModify x 241 ops/sec ±10.27% (61 runs sampled)
Update - Mongoose - find and update, separate ops x 237 ops/sec ±8.71% (60 runs sampled)

read

Read - Mongoose - Basic x 584 ops/sec ±2.13% (85 runs sampled)
Read - Driver - Basic x 691 ops/sec ±0.74% (89 runs sampled)
Read - Mongoose - With lean x 630 ops/sec ±0.86% (88 runs sampled)
Read - Mongoose - Multiple Items x 170 ops/sec ±7.15% (62 runs sampled)
Read - Driver - Multiple Items x 506 ops/sec ±2.16% (88 runs sampled)
Read - Mongoose - Non-index x 202 ops/sec ±6.41% (63 runs sampled)
Read - Driver - Non-index x 453 ops/sec ±1.67% (81 runs sampled)
Read - Mongoose - Embedded Docs x 308 ops/sec ±7.60% (69 runs sampled)
Read - Driver - Embedded Docs x 667 ops/sec ±1.09% (85 runs sampled)

casting

Casting - Embedded Docs - 0 Docs x 2,868 ops/sec ±10.59% (46 runs sampled)
Casting - Embedded Docs - 10 Docs x 3,336 ops/sec ±7.79% (52 runs sampled)
Casting - Embedded Docs - 100 Docs x 3,043 ops/sec ±9.36% (48 runs sampled)
Casting - Embedded Docs - 1000 Docs x 1,864 ops/sec ±11.40% (41 runs sampled)
Casting - Embedded Docs - 10000 Docs x 469 ops/sec ±11.38% (38 runs sampled)

population

Populate - 1 value x 523 ops/sec ±3.84% (88 runs sampled)
Populate - 10 values x 339 ops/sec ±6.42% (77 runs sampled)
Populate - 100 values x 64.85 ops/sec ±6.45% (52 runs sampled)
Populate - 1000 values x 11.28 ops/sec ±7.97% (38 runs sampled)
Populate - 10000 values x 3.11 ops/sec ±3.93% (20 runs sampled)
Populate - 5 properties x 12.64 ops/sec ±7.48% (39 runs sampled)
Populate - 10 properties x 9.97 ops/sec ±7.59% (49 runs sampled)

multiop

Multi-Op - Mongoose - Heavy Read, low write x 7.32 ops/sec ±6.02% (39 runs sampled)
Multi-Op - Driver - Heavy Read, low write x 12.88 ops/sec ±4.26% (37 runs sampled)
Multi-Op - Mongoose - Embedded Docs - Heavy Read, low write x 1.70 ops/sec ±8.14% (13 runs sampled)
Multi-Op - Driver - Embedded Docs - Heavy Read, low write x 10.52 ops/sec ±3.92% (52 runs sampled)
Multi-Op - Mongoose - Heavy Write, low read x 4.14 ops/sec ±6.26% (25 runs sampled)
Multi-Op - Driver - Heavy Write, low read x 8.34 ops/sec ±3.04% (43 runs sampled)
Multi-Op - Mongoose - Embedded Docs - Heavy Write, low read x 0.60 ops/sec ±6.92% (8 runs sampled)
Multi-Op - Driver - Embedded Docs - Heavy Write, low read x 5.04 ops/sec ±4.93% (29 runs sampled)
Multi-Op - Mongoose - Embedded Docs - Read-write-update x 1.33 ops/sec ±5.11% (11 runs sampled)
Multi-Op - Driver - Embedded Docs - Read-write-update x 8.54 ops/sec ±3.72% (44 runs sampled)

[aheckmann]

merged. Thanks!