Fixing issue with multi-line fields (only first line was captured)

[bramadams]

When invoking consult-mu-contacts while being in a mu4e message buffer, I received an error message due to the "To:" field spanning more than one line, like:

To: "A B" [email protected], "E F" [email protected]

Basically, consult-mu--message-get-header-field only considered the line containing "To:" instead of the potential subsequent lines. This pull request is addressing this by scanning for all lines following a given field that do not contain a ":", joining those lines into one string without newlines.