Fix stacktraceusage with more than 1 rule

[304NotModified]

This was broken due to too aggressive caching.

This was working:

<nlog>
   <targets>
       <target name='debug' type='Debug' layout='${message}' />
       <target name='debug2' type='Debug' layout='${callsite} ${message}' />
   </targets>
   <rules>
       <logger name='*' minlevel='Debug' writeTo='debug2' />
       <logger name='*' minlevel='Debug' writeTo='debug' />
   </rules>
</nlog>

and this wasn't (nullref in stacktrace renderer, because of maxstacktrace = none)

<nlog>
   <targets>
       <target name='debug' type='Debug' layout='${message}' />
       <target name='debug2' type='Debug' layout='${callsite} ${message}' />
   </targets>
   <rules>
       <logger name='*' minlevel='Debug' writeTo='debug' />
       <logger name='*' minlevel='Debug' writeTo='debug2' /> 
   </rules>
</nlog>

fixes #1363

[codecov-io]

Current coverage is 75.18%

Merging #1364 into master will increase coverage by +0.01% as of e60a2c9

@@            master   #1364   diff @@
======================================
  Files          267     267       
  Stmts        15526   15528     +2
  Branches      1631    1631       
  Methods          0       0       
======================================
+ Hit          11672   11674     +2
  Partial        383     383       
  Missed        3471    3471       

Review entire Coverage Diff as of e60a2c9

---

Uncovered Suggestions

1. +0.09% via ...gingConfiguration.cs#374...387
2. +0.09% via ...gingConfiguration.cs#340...353
3. +0.08% via ...argets/FileTarget.cs#1713...1725
4. See 7 more...

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