Converter sometimes expands URI incorrectly

[glass-ships]

(using prefixmaps version 2.4)

I'm not sure if there are other examples of this, or if MGI is unique, but.

If, for example, I want to expand MGI:3696575, it returns the following:

>>> from prefixmaps import load_converter
>>> converter = load_converter("merged")
>>> converter.expand("MGI:3696575")
'https://identifiers.org/MGI/3696575'

(with a slash rather than a colon)

rather than the expected https://identifiers.org/MGI:3696575 (which successfully links to the relevant page)

See also: monarch-initiative/monarch-app#743

[glass-ships]

Possibly related to #78