Fix parsing double type without decimal point

[nallwhy]

InfluxDB can send double type values without a decimal point.
To handle this, use Float.parse/1 instead of String.to_float/1.

[coveralls]

Coverage increased (+1.0%) to 74.221% when pulling d135a1d on nallwhy:fix_parsing_double_type_without_decimal_point into 6fff364 on mneudert:master.

[coveralls]

Coverage remained the same at 73.247% when pulling cd4d615 on nallwhy:fix_parsing_double_type_without_decimal_point into 6fff364 on mneudert:master.

[Ma233]

@nallwhy Hi, would you mind to fix the format? And add some tests like this https://github.com/mneudert/instream/pull/70/files#diff-f89c196f7418cccba10c7749d9fd1311a5ed164bab5338ec8a1a043efcfaad42

[nallwhy]

@nallwhy Hi, would you mind to fix the format? And add some tests like this https://github.com/mneudert/instream/pull/70/files#diff-f89c196f7418cccba10c7749d9fd1311a5ed164bab5338ec8a1a043efcfaad42

Thanks a lot!

[mneudert]

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