Avoid redundantly calling em-http-request middleware

This appears to fix #899. em-http-request is such a ball of spaghetti
that I have my doubts, but the tests are green.

The patch is based on the observation that the em-http-request call
sequence goes:

1. `HttpConnection#activate_connection`
2. `HttpClient#connection_completed`
3. `HttpClient#send_request`

The `#connection_completed` method is normally responsible for calling
the request middleware before passing the adjusted headers/body to
`#send_request` (cf.
https://github.com/igrigorik/em-http-request/blob/34919d760b940dfd27159bbebce5c4dac9845eb6/lib/em-http/client.rb#L53-L62).
But webmock needs the request signature all the way back in
`#activate_connection`, which we want to compute using the
middleware-adjusted headers/body.

So instead, we let `#activate_connection` handle building the request
signature, whose result will be memoized. Then when we monkey patch
`#connection_completed` to look up the headers/body from that signature
without recomputing the values.

This required some tweaking to `#build_request_signature`, where it was
actually failing to follow the same algorithm as em-http-request.
Basically it just means calling em-http-request's internal
`#build_request` method + making sure not to clobber the `headers` local
variable (which was probably not intentional in the first place).

lib/webmock/http_lib_adapters/em_http_request_adapter.rb

@@ -99,6 +99,11 @@ def setup(response, uri, error = nil)
         end
       end
 
+      def connection_completed
+        @state = :response_header
+        send_request(request_signature.headers, request_signature.body)
+      end
+
       def send_request(head, body)
         WebMock::RequestRegistry.instance.requested_signatures.put(request_signature)
 
@@ -164,7 +169,7 @@ def build_webmock_response
       end
 
       def build_request_signature
-        headers, body = @req.headers, @req.body
+        headers, body = build_request, @req.body
 
         @conn.middleware.select {|m| m.respond_to?(:request) }.each do |m|
           headers, body = m.request(self, headers, body)
@@ -178,8 +183,6 @@ def build_request_signature
 
         body = form_encode_body(body) if body.is_a?(Hash)
 
-        headers = @req.headers
-
         if headers['authorization'] && headers['authorization'].is_a?(Array)
           headers['Authorization'] = WebMock::Util::Headers.basic_auth_header(headers.delete('authorization'))
         end

spec/acceptance/em_http_request/em_http_request_spec.rb

@@ -71,6 +71,35 @@ def request(client, head, body)
           end
         end
 
+        it "only calls request middleware once" do
+          stub_request(:get, "www.example.com")
+
+          middleware = Class.new do
+            def self.called!
+              @called = called + 1
+            end
+
+            def self.called
+              @called || 0
+            end
+
+            def request(client, head, body)
+              self.class.called!
+              [head, body]
+            end
+          end
+
+          EM.run do
+            conn = EventMachine::HttpRequest.new('http://www.example.com/')
+            conn.use middleware
+            http = conn.get
+            http.callback do
+              expect(middleware.called).to eq(1)
+              EM.stop
+            end
+          end
+        end
+
         let(:response_middleware) do
           Class.new do
             def response(resp)
@@ -119,6 +148,33 @@ def response(resp)
         context 'making a real request', net_connect: true do
           before { WebMock.allow_net_connect! }
           include_examples "em-http-request middleware/after_request hook integration"
+
+          it "only calls request middleware once" do
+            middleware = Class.new do
+              def self.called!
+                @called = called + 1
+              end
+
+              def self.called
+                @called || 0
+              end
+
+              def request(client, head, body)
+                self.class.called!
+                [head, body]
+              end
+            end
+
+            EM.run do
+              conn = EventMachine::HttpRequest.new(webmock_server_url)
+              conn.use middleware
+              http = conn.get
+              http.callback do
+                expect(middleware.called).to eq(1)
+                EM.stop
+              end
+            end
+          end
         end
 
         context 'when the request is stubbed' do