Fix KeyError with missing Host in HTTP::Request

When calling `HTTP::Client#exec` with an `HTTP::Request` instance the
Host header is not automatically filled by the http client, so we need
to add it manually or the `host_matches?` check in the stub will fail
with a `KeyError` exception.

spec/webmock_spec.cr

@@ -341,6 +341,16 @@ describe WebMock do
     end
   end
 
+  it "works with exec" do
+    WebMock.wrap do
+      WebMock.stub(:get, "http://www.example.com")
+
+      client = HTTP::Client.new "www.example.com"
+      request = HTTP::Request.new("GET", "/")
+      client.exec request
+    end
+  end
+
   describe ".calls" do
     it "returns 0 by default" do
       WebMock.wrap do

src/webmock/core_ext.cr

@@ -8,6 +8,7 @@ end
 class HTTP::Client
   private def exec_internal(request : HTTP::Request)
     request.scheme = "https" if tls?
+    request.headers["Host"] = host_header unless request.headers.has_key?("Host")
     stub = WebMock.find_stub(request)
     return stub.exec(request) if stub