set custom user-agent on all requests

[rtrox]

This PR sets a custom user-agent on all requests set by the speedtest library: showwin/speedtest-go <version>. This UserAgent can be overridden with a new Option, WithUserAgent. This WithUserAgent option is just a convenience function -- it wraps the transport on the client's Doer to add a user-agent to all requests. As such, you can use it with a custom Doer if you want (as long as you pass the custom doer first), or you can just implement your own User-Agent on the doer you pass in.

I thought this might be a little nicer way to handle user-agents, but this PR is definitely a little opinionated, glad to adjust or close, just lmk :-)

New Unit Tests are added to test the user-agent behavior, and all tests pass:

 ~/speedtest-go ❯❯❯ go test ./... -v
?       github.com/showwin/speedtest-go [no test files]
=== RUN   TestDownloadTestContext
--- PASS: TestDownloadTestContext (0.60s)
=== RUN   TestDownloadTestContextSavingMode
--- PASS: TestDownloadTestContextSavingMode (0.60s)
=== RUN   TestUploadTestContext
--- PASS: TestUploadTestContext (0.60s)
=== RUN   TestUploadTestContextSavingMode
--- PASS: TestUploadTestContextSavingMode (0.60s)
=== RUN   TestFetchServerList
--- PASS: TestFetchServerList (0.17s)
=== RUN   TestDistance
--- PASS: TestDistance (0.00s)
=== RUN   TestFindServer
--- PASS: TestFindServer (0.00s)
=== RUN   TestNew
=== RUN   TestNew/DefaultDoer
=== RUN   TestNew/CustomDoer
--- PASS: TestNew (0.00s)
    --- PASS: TestNew/DefaultDoer (0.00s)
    --- PASS: TestNew/CustomDoer (0.00s)
=== RUN   TestUserAgent
=== RUN   TestUserAgent/DefaultUserAgent
=== RUN   TestUserAgent/CustomUserAgent
=== RUN   TestUserAgent/CustomUserAgentAndDoer
--- PASS: TestUserAgent (0.00s)
    --- PASS: TestUserAgent/DefaultUserAgent (0.00s)
    --- PASS: TestUserAgent/CustomUserAgent (0.00s)
    --- PASS: TestUserAgent/CustomUserAgentAndDoer (0.00s)
=== RUN   TestFetchUserInfo
--- PASS: TestFetchUserInfo (0.05s)
PASS
ok      github.com/showwin/speedtest-go/speedtest       2.628s

Note: to access the speedtest-go version, I had to move it to speedtest.go, so if you like this behavior, new releases will need to bump the version there instead.

[showwin]

@rtrox Thank you for making it easy to change 😄

@mikaelpeltier Hi✋ The way to add HTTP Header is changed by this PR, please take care about this change 🙏

[rtrox]

@mikaelpeltier - the default user agent is changed, but with this change, there's still a user agent set on all requests, so I don't think it should regress the issue you reported. If it does, lemme know and I'm glad to debug!

[r3inbowari]

Hi @rtrox. It seems that WithDoer and WithUserAgent can not be used at the same time, because WithDoer will always be executed after WithUserAgent in New(opts ...Option).