requests

If you're using proxies with requests-html and rendering JS sites is all good. Once you render a website pyppeteer don't know about this proxies and will expose your IP. This is an undesired behavior when scraping with proxies.

The idea is that whenever someone passes in proxies to the session object or any method call, make pyppeteer also use these proxies. #265

PSR的RequestInterface没有规定实现exec方法，所以我设计组装一个http客户端无关的request的时候，我肯定不能绑定和客户端强相关的exec方法到Request上，因为每个客户端的异常类型、处理逻辑都不相同。

我设计一个composer组件，在组装请求部分，返回了个psr对象，本意是guzzle或者saber等支持PSR标准的HTTP客户端都可以按psr标准把这个对象代表的请求发送出去，现在Guzzle可以做到（$guzzleClient->send($PSRrequest)）而saber因为把PSR相关的处理逻辑绑定到他自定义的Request上，导致这样的设计没法实施。

Is your feature request related to a problem? Please describe.
Related to a problem surfaced by @Shanoir on Gitter.

Describe the solution you'd like
I think that it would be a good idea for us to let users set the filename and content-type headers through the Part annotation. Maybe like so:

    @multip