aria2c doesn't support writing remote content to stdout

[yyuu]

I've implemented aria2c support in #904. However, I finally realized that aria2c doesn't support writing remote content to stdout (aria2/aria2#190).

I confirmed that basically this must be not a problem for ruby-build since it doesn't rely on http get without filename (the only usage of http get is specifying its destination filename). It'd be better to be fixed though.

[mislav]

Thanks for catching this. Like my comment in #904 (comment), I don't like that every tweak to aria2c support needs to touch all tests. Is there a way we can circumvent this? Maybe go back to stubbing curl instead, and aria2c as uninstalled on the system, and then have just one test that stubs aria2c as supported and tests its invocation?

[yyuu]

It sounds nicer than current test impl..... I'll try to do it later.

[yyuu]

I've prepared test helper to remove aria2c from $PATH. Now we can use curl stub during test as same as before. I've prepared dedicated test case for aria2c and it'd be sufficient to check if aria2c is working.

@mislav could you review the changes again?

[mislav]

Thanks for doing all the work and being patient!