[v6] improve contentType detection in overloaded methods

[chradek]

@jeremymeng found an issue where when an operation has overloads, contentType is misidentified in the arguments list.

The reason for this is because previously, we were detecting which argument is the contentType by looking at the code model's operation.requests array and getting the index from that. However, if all the overloads share parameters, those parameters are on operation.parameters. This meant that if a parameter from operation.parameters showed up in the overload before the ones in operation.requests[i].parameters, the contentType lookup would be incorrect.

In some cases (though I haven't been able to identify when) this actually does occur. For example, in form-recognizer. Interestingly when I tried copying the operation that had an issue in form-recognizer into a new swagger model and generated a client for it, contentType was always the first parameter. I was able to verify this change fixes the issue by regenerating form-recognizer.

You can see the issue in the generated code here:

After this change, args[1] is correctly checked in the if/else statements for contentType.

[joheredi]

Looks good!