Removing the "If-Modified-Since" request header when the cache is empty (as it causes issues)

[divdavem]

octokat failed today on my machine because GitHub returned a 304 status code when the cache was empty. I think that sending an "If-Modified-Since" header is wrong when we don't expect a 304 response status. Removing the "If-Modified-Since" header in the request fixed my issue, that's why I am opening this pull request

Here is a js file with which you can reproduce the issue on node.js (after making sure the co and octokat npm packages are installed):

"use strict";

const co = require("co");
const Octokat = require("octokat");

co(function * () {
    const octokat = new Octokat({
       // note that I used a token here because my ip was otherwise rate-limited 
    });
    const response = yield octokat.repos("ariatemplates", "ariatemplates").commits.fetch({
        sha: "master"
    });
    console.log(response);
}).catch(function (error) {
    console.log("Error", error);
});

Here is the output I got:

D:\git_clones\t\test\node_modules\octokat\dist\node\plugins\cache-handler.js:51
            throw new Error('ERROR: Bug in Octokat cacheHandler. It had an eTag but not the cached response');
            ^

Error: ERROR: Bug in Octokat cacheHandler. It had an eTag but not the cached response
    at CacheHandler.module.exports.CacheHandler.responseMiddleware (D:\git_clones\t\test\node_modules\octokat\dist\node\plugins\cache-han
dler.js:51:19)
    at Object.instance._parseWithContext (D:\git_clones\t\test\node_modules\octokat\dist\node\base.js:124:39)
    at D:\git_clones\t\test\node_modules\octokat\dist\node\requester.js:207:38
    at xhr.onreadystatechange (D:\git_clones\t\test\node_modules\octokat\dist\node\requester.js:37:18)
    at dispatchEvent (D:\git_clones\t\test\node_modules\xmlhttprequest\lib\XMLHttpRequest.js:591:25)
    at setState (D:\git_clones\t\test\node_modules\xmlhttprequest\lib\XMLHttpRequest.js:610:14)
    at IncomingMessage.<anonymous> (D:\git_clones\t\test\node_modules\xmlhttprequest\lib\XMLHttpRequest.js:447:13)
    at emitNone (events.js:72:20)
    at IncomingMessage.emit (events.js:166:7)
    at endReadableNT (_stream_readable.js:903:12)

I tried with version 0.5.0-beta.0 of octokat but previous versions also failed in a different way (returning an empty string instead of an error).

Note that I have successfully used the octokat package several times with requests similar to this one, so this issue probably comes from a change on GitHub servers.

Thank you in advance for taking this pull request into account.
Thank you for the great work you are doing by providing octokat to the community!

[marnen]

Please consider merging this. The If-Modified-Since header does indeed seem to be causing problems.

[philschatz]

Thank you for fixing this, these changes will be in the next release!