Update schema mismatch error to include type string

[JeremyPlease]

This is a really small change to the schema mismatch error message string.

Example schema:

{
    "className": "Rocket",
    "fields": {
        "launchTime": { "type": "Date" },
    }
}

Attempted bad request:
rocket.save({ launchTime: 'never!' })

Error response before this pull request:
"schema mismatch for Rocket.launchTime; expected [object Object] but got String"

Error response after this pull request:
"schema mismatch for Rocket.launchTime; expected Date but got String"

[codecov-io]

Current coverage is 91.79%

Merging #1898 into master will increase coverage by <.01%

@@             master      #1898   diff @@
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  Files            91         91          
  Lines          6396       6397     +1   
  Methods        1105       1105          
  Messages          0          0          
  Branches       1344       1345     +1   
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+ Hits           5871       5872     +1   
  Misses          525        525          
  Partials          0          0          

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[drew-gross]

Awesome, thank you!