refactor(core): replace add with setContentType in Request.Builder for header consistency

Author: predic8

core/src/main/java/com/predic8/membrane/core/http/Request.java

@@ -257,7 +257,7 @@ public Builder header(String headerName, String headerValue) {
         }
 
         public Builder contentType(String value) {
-            req.getHeader().add(CONTENT_TYPE, value);
+            req.getHeader().setContentType( value);
             return this;
         }