Quoted boundary handling for multipart

[tlethanh-sdi]

Some implementations (ex: Qt framwork) used the quoted boundary for multipart form data. Actually, we need a patch for the set_boundary() function in order to handle the multipart data correctly.
It should be nice if cpp-httplib handles this, as it is a part of the standard.

The reference of the quoted boundary is described here: https://tools.ietf.org/html/rfc2046#section-5.1.1

Here is the patch we used:
`

	void set_boundary(std::string boundary) {

		//Remove double-quote from the input string.
		if( boundary.length() >= 2 && boundary.front() == '"' && boundary.back() == '"' )
		{
			boundary_ = boundary.substr( 1, boundary.size() - 2 );
		}else {
			boundary_ = boundary;
		}
	}

`

[yhirose]

@tlethanh-sdi, I fixed it. Could you try with the latest master?

[tlethanh-sdi]

@yhirose Great thanks to you. The code is now on our test chain. I will confirm the result very soon.

[tlethanh-sdi]

@yhirose : I just want to confirm that the code works. Thanks.