Fix ambiguous mapping of metadata vs pattern collections

[webketje]

What should the resulting collection property on file metadata look like in the following separate/ mixed configs?
Below is the current config

# collections / location	front-matter	pattern	both
1	'col1'	'col1'	[['col1'],'col2']
more	['col1','col2']	['col1','col2']	['col1','col2']

There are 2 bugs in 1.2.0:

• (minor) collection names are not sorted, file metadata collections will come first
• (major) when multiple collections are added, they will unfortunately be nested arrays

To fix this: add unit tests with multiple mixes of this matrix.

Question: is it safe to assume that users would want pattern- & frontmatter-based collections to be merged? Answer: #32