fix: 937 - removed home-made user agent header when for web

[monsieurtanuki]

What

• Our home-made user agent prevents our dart package to work when used in web mode.
• We use this specific user agent for stats.
• The solution implemented here removes the user agent http header when in web mode, so that the package can be used.
• Possible improvements would be to change the way we build our user agent string (but that would have an impact on our previous/next stats), or where we put that home-made string (e.g. different http header tag) (but that would have an impact on our stats consistency too)
• We'll see how many requests we get from web mode. Later we can select one of the suggested improvements, or something else.

Fixes bug(s)

• Fixes: Flutter Web and the User Agent #937

Impacted files

• api_add_product_image_test.dart: minor refactoring
• http_helper.dart: removed home-made user agent header when for web; minor refactoring
• open_prices_api_client.dart: minor refactoring
• uri_reader.dart: added an isWeb getter; minor refactoring
• uri_reader_js.dart: implemented the isWeb getter