Fixing set config.method after mergeConfig for Axios.prototype.request (#2383)

Inside Axios.prototype.request function, It's forced to set
method to 'get' after `mergeConfig` if config.method exists, which makes the defaults.method not effective.

lib/core/Axios.js

@@ -35,7 +35,15 @@ Axios.prototype.request = function request(config) {
   }
 
   config = mergeConfig(this.defaults, config);
-  config.method = config.method ? config.method.toLowerCase() : 'get';
+
+  // Set config.method
+  if (config.method) {
+    config.method = config.method.toLowerCase();
+  } else if (this.defaults.method) {
+    config.method = this.defaults.method.toLowerCase();
+  } else {
+    config.method = 'get';
+  }
 
   // Hook up interceptors middleware
   var chain = [dispatchRequest, undefined];