Operation spec missing `contentType` field when `"consumes" = [ "application/json" ]` override is specified

[jeremymeng]

(more detailed discussion at https://github.com/Azure/azure-sdk-for-js/pull/5980#discussion_r348009598)

Short summary

• the unified swagger has "consumes" = [ "application/xml" ] globally
• the file create permission operation overrides it to "consumes" = [ "application/json" ]
• the overwrite caused generated createPermissionOperationSpec to miss the contentType field.
• Regardless, the request is then sent with contentType of text/plain;charset=UTF-8, which caused failure.

It looks like autorest typescript plugin should generate correct contentType field when "consumes" = [ "application/json" ] is specified.

The same swagger file has another consumes override to application/octa-stream

https://github.com/Azure/azure-rest-api-specs/blob/7fd37fcd3c54742d07bd70435f75ef46ec99d9e3/specification/storage/data-plane/Microsoft.FileStorage/preview/2019-02-02/file.json#L2797

and it generates operation spec with expected contentType field:

https://github.com/Azure/azure-sdk-for-js/blob/cf0c4bc079f5890d520124aad92af6630e5a1966/sdk/storage/storage-file-share/src/generated/src/operations/file.ts#L717

[jeremymeng]

@sarangan12 @daviwil

[joheredi]

We need to verify if this also happens in v6