Fix JS Generator Identifier when name is camel case

[nnnnoel]

This PR fixes JS Generator when field name is camel case
protocolbuffers/protobuf-javascript#17

example

message Message {
    int32 someCamelCaseMessage = 1;
}

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[nnnnoel]

@acozzette can you review about this?

[perezd]

@lukesandberg is responsible for JS, and you'll need to sign the CLA before we can continue w/ review.

[nnnnoel]

@perezd I signed CLA 7 days ago

[nnnnoel]

moved to protocolbuffers/protobuf-javascript#3