Add JUnit Pioneer and use it in ServerErrorHealthCheck

[sleberknight]

See my original comment here: #53 (comment)

[sleberknight]

Added Pioneer and then attempted to use @DoubleRangeSource in ServerErrorHealthCheckTest. Alas, got reminded why floating point math sucks, because we do calculations with the input values to mock out the rate for the Meter, and we include the approximate count in the health check details. In any case you end up with errors like this:

java.lang.AssertionError: [Expected detail not found] 
Expecting map:
 <{"approximateCount"=0.8999999999999998, "criticalThreshold"=10, "meter"="io.dropwizard.jetty.MutableServletContextHandler.5xx-responses", "rate"=9.999999999999998E-4, "severity"="OK", "warningThreshold"=1}>
to contain:
 <[MapEntry[key="approximateCount", value=0.8999999999999999]]>
but could not find the following map entries:
 <[MapEntry[key="approximateCount", value=0.8999999999999999]]>

Without changing the code in the health check, this cannot work. For example it would work if instead we only included the first few digits in the details. But in order to do that with a double, we'd need to either use BigDecimal or use something like String.format("%.1f", value).

Do we want to do this? For example using .withDetail("approximateCount", String.format("%.1f", estimatedErrorCount)) for the above value (0.8999999999999998) would then result in a detail of ("approximateCount", 0.9)

[sleberknight]

Actually the above is wrong...the detail's value would be the String "0.9" rather than a number. So we'd have to convert it back to a double to be semantically correct.

[sleberknight]

So it would need to be:

.withDetail("approximateCount", Double.parseDouble(String.format("%.1f", estimatedErrorCount)))

There's probably some much more sophisticated math way to do this, but...

[sleberknight]

Decided to go ahead with this change, using the code above to set the approximateCount detail.