Chained template parsing failed if parent is "html.erb" and child is "html.md.erb"

Given the following templates:

* Parent template "parent.html.erb"
* Child template "child.html.md.erb"

I found an unexpected behaviour of `middleman`: The child template is not compiled correctly. Just the Erb-part is compiled, the markdown part is not compiled. Is this behaviour intended or is it a bug or did I do something wrong?

I added a test to clarify what I mean. This is why I opened an PR. The test failed with:
```
Scenario: Partials are parsed by multiple template engines       # features/chained_templates.feature:21
  Given the Server is running at "partial-chained_templates-app" # lib/middleman-core/step_definitions/server_steps.rb:61
  When I go to "/parent.html"                                    # lib/middleman-core/step_definitions/server_steps.rb:66
  Then I should see:                                             # lib/middleman-core/step_definitions/server_steps.rb:86
    """
    <h1>Parent</h1>
    """
  Then I should see:                                             # lib/middleman-core/step_definitions/server_steps.rb:86
    """
    <h2 id="child">Child</h2>
    """
    expected "<h1>Parent</h1>\n\n## Child\n\nhello world\n\n" to include "<h2 id=\"child\">Child</h2>"
    Diff:
    @@ -1,2 +1,6 @@
    -<h2 id="child">Child</h2>
    +<h1>Parent</h1>
    +
    +## Child
    +
    +hello world
     (RSpec::Expectations::ExpectationNotMetError)
    ./lib/middleman-core/step_definitions/server_steps.rb:87:in `/^I should see:$/'
    features/chained_templates.feature:28:in `Then I should see:'
  Then I should see:                                             # lib/middleman-core/step_definitions/server_steps.rb:86
    """
    <p>hello world</p>
    """
```

Thank you very much.