SuspectTransform: reduce false positives in arithmetic evaluation

[brendan-rsoc]

We've noticed that websites with a lot of numbers on them tend to falsely trigger detections for arithmetic evaluation. This PR reduces false positive alerts by requiring arithmetic evaluation detection to meet the following criteria:

• Result must not be preceded by a decimal point or another digit
• Result must not be followed by another digit

Example of new behavior:

Request: 
1111*4

Response: 
123.4444 # Not a match
1244445567 # Not a match
content content 4444 content # Match