[Ruby] Allow Overriding Accept Header

[ckoegel]

The Ruby SDK currently does not allow for overriding the Accept header at runtime. The header can be set using the header_params argument when calling an operation method, but this gets overwritten by the select_header_accept method called when the data is serialized to a request. This PR adds a conditional that skips the calling of the select_header_accept method if an Accept header has been provided by the user.

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@cliffano (2017/07) @zlx (2017/09) @autopp (2019/02)

[wing328]

lgtm. thanks for the PR