Handle redirects from vault server versions earlier than v0.6.2

[drmdrew]

Fix issue #75.

There are two golang HTTP related issues that are involved here, due to the way golang 1.7 http package reacts to HTTP redirects:

• net/http: copy headers when following redirects golang/go#4800
• net/http: API provides no way to follow 307 redirect for PUT or POST golang/go#7912 or net/http: 301 redirects after a 303 redirect from a POST request don't work golang/go#9348

The first issue causes the X-Vault-Token header to be dropped when following redirects of HTTP GET requests. The second issue causes redirects on POST requests not to be followed at all.

It does look like golang 1.8 will address both these issues once it is released... so these fixes are just needed in the meantime.

TODO:

• manual testing with vault HA-server configuration w/redirects
• updated tests

PTAL: @hairyhenderson

[hairyhenderson]

@drmdrew can you run gofmt -s -w . on this? As well as make sure that golint ./... passes. Otherwise LGTM

[drmdrew]

Re-based to fixup gofmt and golint issues.

[filiptodoric]

You guys are the best 👍