Fix header url param to not render html to resolve XSS

[elongstreet88]

Fixes https://github.com/billchurch/webssh2/issues/345 http://localhost:2222/ssh/host/mydevice.local?header=<img src=x onerror=alert('XSS')>

Before:

After:

Note - This could be breaking if someone is using the header for HTML rendering, however, i would say this is still justified.

[sonarqubecloud[bot]]

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