Steps to reproduce:

1. Create new item ("Numeric (unsigned)") with "Preprocessing steps": "Jsonpath"

   $.bignumber
   

2. Press "Test all steps" and supply value with

   {"bignumber":1504135699244380}
   

3. Press "Test"

Result:
We will receive "Error"

Value of type "string" is not suitable for value type "Numeric (unsigned)". Value "1.50413569924438E+15"

Expected:
We will receive without "Error"

1504135699244380

P.s:
If we supply a bit increased value (1504135699244380+1 = 1504135699244381)

{"bignumber":1504135699244381}

We will receive (as we should):
1504135699244381