fix: prevent options merging fn exec without default value

[benjamincanac]

Functions in configuration are meant to allow merging default values. Now, functions are only executed if the value already exists in the configuration.

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

When using extendParser with a function '.txt': (file) => ({ body: file }), the function was executed.

[mathe42]

So if we want to overwrite an existing parser we have to use:

{
  '.xml': file => parse(file)
}

For a new extension we can use:

{
  '.custom': file => parse(file)
}

~

[codecov-commenter]

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[mathe42]

Maybe we can add one sentence in the docs for this...

[mathe42]

Never mind I was wrong...

[mathe42]

Looks good to me.