Bugfix for code generation while using goField and goModel tags together for one type.

[atzedus]

Example code:

type AdFormatAlias @goModel(forceGeneration: true) {
    id: ID!
    name: String!
    organization: Organization! @goField(forceResolver: true)
}

In this combination @GoModel tag is loosing, because map entry fully rewrited without copy property ForceGenerate.

This PR fixes this issue and (hopefully) prevents such problems in the future.

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

[coveralls]

coverage: 73.535% (-0.06%) from 73.597%
when pulling 26e7311 on atzedus:bugfix_for_go_field_tag_generation
into 5310d08 on 99designs:master.