Remove explicit WriteHeader call to have correct content type

According to the documentation:

    // WriteHeader sends an HTTP response header with status code.
    // If WriteHeader is not called explicitly, the first call to Write
    // will trigger an implicit WriteHeader(http.StatusOK).
    // Thus explicit calls to WriteHeader are mainly used to
    // send error codes.

and

    // Header returns the header map that will be sent by WriteHeader.
    // Changing the header after a call to WriteHeader (or Write) has
    // no effect.

so calling `w.Header().Set(contentType, jsonContentType)` after calling
`w.WriteHeader` does not set the content type -- the call can be removed
though as it always set `http.StatusOK` which will be done anyways

registry.go

@@ -128,7 +128,6 @@ func (registry *Registry) YieldExporter() http.HandlerFunc {
 			url := r.URL
 
 			if strings.HasSuffix(url.Path, jsonSuffix) {
-				w.WriteHeader(http.StatusOK)
 				w.Header().Set(contentType, jsonContentType)
 				composite := make(map[string]interface{}, len(registry.NameToMetric))
 				for name, metric := range registry.NameToMetric {