Clean up micropub creation spec

What I was trying to do with the regex is figure out if location header
pointed at the micropost. A regex isn't needed for that, just an `eq`
matcher.

Also, I found a bug in that the location header needed to point toward
the full url, not just the path which is something a regex wouldn't have
noticed!

app/controllers/api/micropubs_controller.rb

@@ -3,6 +3,6 @@ class Api::MicropubsController < ApplicationController
 
   def create
     micropost = Micropost.create(body: params[:content])
-    head :created, location: micropost_path(micropost)
+    head :created, location: micropost_url(micropost)
   end
 end

spec/requests/micropub_creation_spec.rb

@@ -19,8 +19,7 @@
 
       expect(Micropost.count).to eq(micropost_count + 1)
       expect(response).to have_http_status(:created)
-      path_matcher = %r(#{microposts_path}/\d+)
-      expect(response.headers["Location"]).to match(path_matcher)
+      expect(response.headers["Location"]).to eq(micropost_url(Micropost.last))
     end
   end
 end