Split header into at most two parts

We receive headers as strings on the form 'key: value', and get both
parts by splitting the string on ':'. We didn't however restrict how
many parts we might split the string into, and the value part might also
contain colons which we should not split on.

Restrict the number of parts we might split the string into to two,
thereby avoid also splitting the value as well.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>