docs(network): use lower-cased header name in delete header samples

[Armin Azar (armin-azar)]

Summary

• request.headers() returns lower-cased header names, so X-Secret never matches and the "Delete header" samples for JavaScript, Java and C# silently leave the header in place instead of removing it.
• Lowercased the key in those three samples so they do what the surrounding text says. The Python samples already used the lower-cased name.

[Armin Azar (armin-azar)]

Max Schmitt (@mxschmitt) mind taking a look when you get a chance? Small one. The delete header samples use X-Secret, but request.headers() returns lower-cased names, so that delete never matches and the header survives. This just lower-cases the key in the JS, Java and C# samples. Python was already correct.