Add HeaderCollection annotation, more cleanup

[ghost]

No description provided.

[ghost]

@RikkiGibson I checked this against the storage repository and the generated code compiles properly. Once this pull request is merged, we can do another release and storage should be good to go.

[olydis]

🤖 AutoRest automatic feature coverage report 🤖

feature set version 2.3.14

❌️ General: 92%

40 out of 522 features are not covered by tests

ConstantsInBody, ConstantsInPath, FileStreamVeryLarge, FormdataStreamUploadFile, HttpRedirect300Get, HttpRedirect300Head, HttpRedirect301Get, HttpRedirect301Put, HttpRedirect302Get, HttpRedirect302Head, HttpRedirect302Patch, HttpRedirect303Post, HttpRedirect307Delete, HttpRedirect307Get, HttpRedirect307Head, HttpRedirect307Patch, HttpRedirect307Post, HttpRedirect307Put, ResponsesScenarioC200MatchingModel, ResponsesScenarioC201MatchingModel, ResponsesScenarioC404MatchingModel, StreamUploadFile, UrlPathsArrayCSVInPath, UrlQueriesArrayMultiEmpty, UrlQueriesArrayMultiNull, UrlQueriesArrayMultiValid, allowedValueEnum, expectedEnum, getByteEmpty, getByteInvalid, getComplexPolymorphismComplicated, getEnumReferenced, getEnumReferencedConstant, getStringMultiByteCharacters, putComplexPolymorphismComplicated, putEnumReferenced, putEnumReferencedConstant, putStringMultiByteCharacters, roundTripEnum, unexpectedEnum

❌️ Azure: 88%

14 out of 118 features are not covered by tests

AzureMethodQueryUrlEncoding, AzureODataFilter, AzurePathQueryUrlEncoding, AzureRequestClientIdInError, AzureSwaggerQueryUrlEncoding, AzureXmsCustomNamedRequestId, AzureXmsCustomNamedRequestIdParameterGroup, AzureXmsRequestClientIdNull, CustomHeaderPostAsyncSucceded, CustomHeaderPostSucceeded, CustomHeaderPutAsyncSucceded, CustomHeaderPutSucceeded, LRONonRetryPut201Creating400InvalidJson, PagingOdataMultiple

[RikkiGibson]

Looks great--like the way you've composed together the different parts of method generation