[Go] Replace .Username with .UserName in generated API methods.

[ivucica]

Right now, any generated code where the http basic authentication is
required is broken, as it tries to access nonexistent field in the
generated Configuration struct.

The alternative is to fix configuration.mustache to replace the
UserName field with Username field.

[ivucica]

It would be great if some Go code would be built as part of CI tests, covering this type of change. What would be a good way of tackling this?

[wing328]

@ivucica totally agree leveraging the CI and Go Petstore API client to catch the issue moving forward.

I'll submit a PR to update the test spec.

[wing328]

I've added test cases via #3574 to cover the issue and got the following error before your fix:

[INFO] --- exec-maven-plugin:1.2.1:exec (go-get-resty) @ Goswagger ---
[INFO] 
[INFO] --- exec-maven-plugin:1.2.1:exec (go-test) @ Goswagger ---
# _/Users/williamcheng/Code/jul2016/swagger-codegen/samples/client/petstore/go/go-petstore
go-petstore/fake_api.go:137: a.Configuration.Username undefined (type Configuration has no field or method Username)
FAIL    _/Users/williamcheng/Code/jul2016/swagger-codegen/samples/client/petstore/go [build failed]

[wing328]

@ivucica FYI. There's a PR to undo the change: #4245.

Let me know if you've concern.

[ivucica]

This is fine. #4245 seems to also be covering my comment:

The alternative is to fix configuration.mustache to replace the
UserName field with Username field.

I do not understand what @paulvollmer's "the UserName variable does not work in CamelCase" means, i.e. why this is necessary, but it seems like a clean fix.