Deep copy options of Fetch instead of modifying original one

[zhouhn-ibm]

WHY: In some rare cases, a global variable is passed through as options of Fetch API. Then weasel modifies the type of headers in the parameter to Headers object during appending Instana correlation header. This case changes the type of a global variable and might cause conflict with further user cases.

Using a deep copy of options of Fetch and keeping the input one untouched is more safe.

WHAT: Update hook of Fetch API to use deep copy of Fetch options.

[zhouhn-ibm]

I replaced Firefox 59 on Win 10 with Firefox 58 on Win 11 as there is a problem on saucelab blocks the original test.