[FIX] Issue #7 : Check products before fetching price

[TheSerapher]

As requested, a quick fix for the current issue with unavailable currencies in the ticker.

Fixes #7

[qoomon]

LGTM

[qoomon]

I just got an answer from the moneymoney support how to do it the right way you have to set the Accept header to application/json then no exception is thrown, even if the api response code is 404. e.g.

local headers = {}
headers["Accept"] = "application/json"
local content = Connection():request("GET", url, nil, nil, headers)
return JSON(content):dictionary()

[leoschweizer]

@TheSerapher having that information from @qoomon now, would you like to implement this differently, or leave it as it is?

[TheSerapher]

I think instead of causing wild 404s and checking for headers and status codes, why not keep it as is and check if we should call it at all.