Better field type detection, fixes #250

[elektro-wolle]

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Performance is slightly worse, especially for Doubles.

Current code:

GelfMessageBenchmark.discoverDoubleField                     avgt    5     34,866 ±   4,471  ns/op
GelfMessageBenchmark.discoverLongField                       avgt    5     15,418 ±   2,406  ns/op
GelfMessageBenchmark.discoverStringField                     avgt    5      5,405 ±   1,440  ns/op

New code:

GelfMessageBenchmark.discoverDoubleField  avgt    5  235,740 ± 5,143  ns/op
GelfMessageBenchmark.discoverLongField    avgt    5   26,484 ± 1,803  ns/op
GelfMessageBenchmark.discoverStringField  avgt    5    5,242 ± 0,254  ns/op

[mp911de]

Thank you for your contribution. That's merged and polished now.