Add support for requestInterceptor / responseInterceptor.

- Display mutated requests from request interceptor in curl output in UI.
  Put this behind showMutatedRequest flag so that the mutation can be
  silent.
- Document requestInterceptor, responseInterceptor and showMutatedRequest
  in README.md
- Add tests

src/core/plugins/spec/actions.js

@@ -12,6 +12,7 @@ export const UPDATE_PARAM = "spec_update_param"
 export const VALIDATE_PARAMS = "spec_validate_param"
 export const SET_RESPONSE = "spec_set_response"
 export const SET_REQUEST = "spec_set_request"
+export const SET_MUTATED_REQUEST = "spec_set_mutated_request"
 export const LOG_REQUEST = "spec_log_request"
 export const CLEAR_RESPONSE = "spec_clear_response"
 export const CLEAR_REQUEST = "spec_clear_request"
@@ -177,6 +178,13 @@ export const setRequest = ( path, method, req ) => {
   }
 }
 
+export const setMutatedRequest = ( path, method, req ) => {
+  return {
+    payload: { path, method, req },
+    type: SET_MUTATED_REQUEST
+  }
+}
+
 // This is for debugging, remove this comment if you depend on this action
 export const logRequest = (req) => {
   return {
@@ -187,8 +195,9 @@ export const logRequest = (req) => {
 
 // Actually fire the request via fn.execute
 // (For debugging) and ease of testing
-export const executeRequest = (req) => ({fn, specActions, specSelectors}) => {
+export const executeRequest = (req) => ({fn, specActions, specSelectors, getConfigs}) => {
   let { pathName, method, operation } = req
+  let { requestInterceptor, responseInterceptor } = getConfigs()
 
   let op = operation.toJS()
 
@@ -207,6 +216,16 @@ export const executeRequest = (req) => ({fn, specActions, specSelectors}) => {
 
   specActions.setRequest(req.pathName, req.method, parsedRequest)
 
+  let requestInterceptorWrapper = function(r) {
+    let mutatedRequest = requestInterceptor.apply(this, [r])
+    let parsedMutatedRequest = Object.assign({}, mutatedRequest)
+    specActions.setMutatedRequest(req.pathName, req.method, parsedMutatedRequest)
+    return mutatedRequest
+  }
+
+  req.requestInterceptor = requestInterceptorWrapper
+  req.responseInterceptor = responseInterceptor
+
   // track duration of request
   const startTime = Date.now()