Fixing problem where if primary key starts with underscore Model.create doesn’t work

[fishcharlie]

Summary:

This PR fixes a problem where if your primary key starts with an underscore Model.create will fail.

Code sample:

General

Cats.Cat12.create({_id: 666, name: 'Garfield'}, function (err, garfield) {
  should.not.exist(err);
  should.exist(garfield);
  done();
});

Type (select 1):

• Bug fix
• Feature implementation
• Documentation improvement
• Testing improvement
• Test added to report bug (GitHub issue #--- @---)
• Something not listed here

Is this a breaking change? (select 1):

• 🚨 YES 🚨
• No
• I'm not sure

Is this ready to be merged into Dynamoose? (select 1):

• Yes
• No

Are all the tests currently passing on this PR? (select 1):

• Yes
• No

Other:

• I have searched through the GitHub pull requests to ensure this PR has not already been submitted
• I have updated the Dynamoose documentation (if required) given the changes I made
• I have added/updated the Dynamoose test cases (if required) given the changes I made
• I have run npm test from the root of the project directory to ensure all tests continue to pass
• I agree that all changes made in this pull request may be distributed and are made available in accordance with the Dynamoose license
• All of my commits and commit messages are detailed, explain what changes were made, and are easy to follow and understand
• I have confirmed that all my code changes are indented properly using 2 spaces
• I have filled out all fields above

[fishcharlie]

@brandongoode Thoughts on this? I'm not sure if _ is the only case we should be looking for here. I tried to skim through the DynamoDB documentation very briefly and couldn't find any other cases we should account for. Maybe it would be safer to try to write a regex to check to see if the first character is not a normal letter?

[coveralls]

Pull Request Test Coverage Report for Build 797

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 85.473%

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Totals
Change from base Build 791:	0.1%
Covered Lines:	2146
Relevant Lines:	2435

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💛 - Coveralls