[api] Fix GraphQL requests made to Enterprise hosts

This invocation would fail:

    GITHUB_HOST=example.com hub api graphql ...

This is because hub would erroneously try to POST to
`example.com/api/v3/graphql` instead of `example.com/api/graphql`.

This applies a workaround that gets rid of the "v3" portion for requests
to "graphql" endpoint.

Fixes #2287

features/api.feature

@@ -280,6 +280,22 @@ Feature: hub api
       {"name":"Jet","size":2}
       """
 
+  Scenario: Enterprise GraphQL
+    Given I am "octokitten" on git.my.org with OAuth token "FITOKEN"
+    Given the GitHub API server:
+      """
+      post('/api/graphql', :host_name => 'git.my.org') {
+        halt 401 unless request.env['HTTP_AUTHORIZATION'] == 'token FITOKEN'
+        json :name => "Ed"
+      }
+      """
+    And $GITHUB_HOST is "git.my.org"
+    When I successfully run `hub api graphql -f query=QUERY`
+    Then the output should contain exactly:
+      """
+      {"name":"Ed"}
+      """
+
   Scenario: Repo context
     Given I am in "git://github.com/octocat/Hello-World.git" git repo
     Given the GitHub API server:

github/http.go

@@ -229,6 +229,11 @@ type simpleClient struct {
 }
 
 func (c *simpleClient) performRequest(method, path string, body io.Reader, configure func(*http.Request)) (*simpleResponse, error) {
+	if path == "graphql" {
+		// FIXME: This dirty workaround cancels out the "v3" portion of the
+		// "/api/v3" prefix used for Enterprise. Find a better place for this.
+		path = "../graphql"
+	}
 	url, err := url.Parse(path)
 	if err == nil {
 		url = c.rootUrl.ResolveReference(url)