Custom converter functions are not recognized for heading tags

[lilacpixel]

When creating custom converters with MarkdownConverter for specific tags, all converters for h1-h6 headings are ignored, and headings are processed according to the standard converter options.

<h1>This is a heading</h1>
<p>This is a paragraph</p>

class FixMarkup(MarkdownConverter):
    def convert_h1(self, el, text, parent_tags):
        return 'Heading override'

def md(html, **options):
    return FixMarkup(**options).convert(html)

markdown = md(html, heading_style='ATX')

Expected output:

Heading override

This is a paragraph

Actual output:

# This is a heading

This is a paragraph

[chrispy-snps]

@lilacpixel - thank you for reporting this. It is indeed a bug; I will fix it.