Handle trailing white spaces in yaws_server:body_method/4

[leoliu]

in accordance with the spec https://greenbytes.de/tech/webdav/rfc7230.html#header.fields:

header-field = field-name ":" OWS field-value OWS

This fix is prompted by the following entry that shows up in report.log from time to time:

=ERROR REPORT==== 3-Apr-2019::18:19:00.139688 === Yaws process died: {badarg, [{erlang,list_to_integer,["0 "],[]}, {yaws_server,body_method,4, [{file, "/myapp/_build/default/lib/yaws/src/yaws_server.erl"}, {line,1737}]}, {yaws_server,aloop,4, [{file, "/myapp/_build/default/lib/yaws/src/yaws_server.erl"}, {line,1261}]}, {yaws_server,acceptor0,2, [{file, "/myapp/_build/default/lib/yaws/src/yaws_server.erl"}, {line,1073}]}, {proc_lib,init_p_do_apply,3, [{file,"proc_lib.erl"},{line,249}]}]}