2.6.0

Start a new support branch v2_ns0m starting from latest v2 tag (2.5.1).

Cherry-pick from parent project main branch some merged Pull Requests:

• Feature silkimen#158: support removing headers which were previously set via "setHeader"
• Fixed silkimen#345: empty file names are not handled correctly (thanks @ikosta)
• Fixed silkimen#359: memory leakage leads to app crashes on Android
• Fixed silkimen#399: memory leakage leads to app crashes on iOS (thanks @avargaskun)
• Fixed silkimen#372: malformed empty multipart request on Android
• Feature silkimen#451: expose response object on downloadFile() (thanks to @MobisysGmbH)
• Fixed silkimen#441: sensible data can be cached in cache.db on iOS (thanks @dtarnawsky)
• Fixed silkimen#429: intermediate CA certs are not respected on iOS when using client certs (thanks @pavrda)

And

• Fixed silkimen#482: remove Accept-Charset header on Android (@ciknowles)