Reinstate 0 Content-Length

A previous commit removed the Content-Length header from communication with
the UAA when negotiating tokens.  This change, while working in most instances
does break some usage.  This change reinstates that header.

[resolves #580]

Author: nebhale

cloudfoundry-client-reactor/src/main/java/org/cloudfoundry/reactor/tokenprovider/AbstractUaaTokenProvider.java

@@ -95,9 +95,10 @@ private Mono<String> getTokenFlow(ConnectionContext connectionContext) {
             .map(this::getTokenUri)
             .then(uri -> connectionContext.getHttpClient()
                 .post(uri, outbound -> outbound
-                    .header(HttpHeaderNames.CONTENT_TYPE, HttpHeaderValues.APPLICATION_X_WWW_FORM_URLENCODED)
-                    .header(HttpHeaderNames.AUTHORIZATION, getAuthorizationValue())
                     .header(HttpHeaderNames.ACCEPT, HttpHeaderValues.APPLICATION_JSON)
+                    .header(HttpHeaderNames.AUTHORIZATION, getAuthorizationValue())
+                    .header(HttpHeaderNames.CONTENT_LENGTH, "0")
+                    .header(HttpHeaderNames.CONTENT_TYPE, HttpHeaderValues.APPLICATION_X_WWW_FORM_URLENCODED)
                     .removeTransferEncodingChunked()
                     .sendHeaders())
                 .doOnSubscribe(NetworkLogging.post(uri))