Update curl.cpp

The space causes signature mismatch when using "ahbe_conf" file to add additional headers.When s3 use the" x-amaz" header to calculates the signature, the format is as follow:
PUT

application/octet-stream
Wed, 05 Nov 2014 03:05:08 GMT
x-amz-acl:private
x-amz-meta-gid:0
x-amz-meta-mode:33188
x-amz-meta-mtime:1415156708
x-amz-meta-uid:0
There is no space after colon.