Readme Parsing gets tripped up by new lines in the headers

[dd32]

A readme.txt that looks something like this:

=== MyPlugin ===
Tags: a, b, c

Stable Tag: 1.2

...

or

=== MyPlugin ===
Tags: a, b, c
Stable Tag: 1.2

MyPlugin: Does these things
...

causes the readme parser to get tripped up and incorrectly parse or discard some of the lines.

In the first case, it will ignore the Stable tag, and in the second it'll discard the short description.

Props Ipstenu for the report.

[dd32]

In 11908:

Plugin Directory: Readme Parser: Be more tolerable of blank lines within the headers if it's followed by a valid header.

This allows for these edgecases to be handled:

• "Tags: a,b,c\n\nStable Tag: 1.2"
• "Tags: a,b,c\nStrangeHeader: true\nStable Tag: 1.2"

along with the expected and valid

• "tags: a,b,c\nStable Tag: 1.2"

while also not discarding a Short description just because it contained a :.

Props Ipstenu for the report.
Fixes #6358.