Define Static File Content Type

[MyNameIsCosmo]

While trying to serve a utf-8 encoded HTML document via Sanic.static, I noticed that content_type cannot be set using Sanic.static.

Sanic header content_type defaults to text/plain
if the type cannot be guessed by mime_types.guess_type, which seems to only analyze the filename.

Currently, the following is not possible (same goes for blueprints):

app.static('/', get_data('index.html'), content_type='text/html; charset=utf-8')

Prior to this PR, you need to do the following to serve a document with a custom content-type:

@app.route('/')
async def handle_index(request):
    return await response.file(
        'index.html',
        headers={'Content-Type': 'text/html; charset=utf-8'}
    )

I'll be submitting a PR shortly with a proposed fix.