http: fix password capture when enabled

The current implementation would only log, if the password contains a
colon, the part before the first colon (e.g., the password
`password:password` would be logged as `password`).

A test has been added to confirm the expected behaviour.

testing/btest/scripts/base/protocols/http/http-basic-auth-colon.zeek

@@ -0,0 +1,8 @@
+# Authorization: Basic password has a colon in its value
+#
+# @TEST-EXEC: zeek -b -r $TRACES/http/basic-auth-with-colon.trace %INPUT
+# @TEST-EXEC: btest-diff http.log
+
+@load base/protocols/http
+
+redef HTTP::default_capture_password = T;