Explictly WriteHeader before Write to avoid superfluous response.Writ…

[WqyJh]

There're some situation when WriteHeader would be called after httpSwagger.Handler which may cause warning of http: superfluous response.WriteHeader call.

Simply add WriteHeader before write data to response solve this problem. The underlying webdav.Handler does the same and won't cause this problem.

Reference to https://github.com/golang/go/blob/72301a9863fb43ff26e9779a086e02cf02031ceb/src/net/http/clientserver_test.go#L1551