Second Round CURL Wrapper Review

[Jakob Ovrum]

The docs for onReceiveHeader,

http://freeze.steamwinter.com/D/web/phobos/etc_curl.html#onReceiveHeader

explicitly says that the const string parameters are not valid 
after the function returns. This is all well and good; but a 
minor improvement would be to change the signature to:

@property void onReceiveHeader(void delegate(in char[] key, in 
char[] value) callback);

to add `scope` to the parameters. This documents the fact that 
the constant strings shouldn't be escaped as-is in both code and 
documentation. I suspect many other instances of `const(char)[]` 
could be changed to `in char[]` to better document how they use 
the parameters (I also personally think it looks better as it is 
more concise).