Fix issue when writeing tag value ending in backslash

fixes #225
ref influxdata/influxdb#5231
ref fangli/fluent-plugin-influxdb#93

Author: dmke

lib/influxdb/point_value.rb

@@ -13,6 +13,7 @@ def initialize(data)
     def dump
       dump =  @series.dup
       dump << ",#{@tags}" if @tags
+      dump << " ".freeze if dump[-1] == "\\"
       dump << " #{@values}"
       dump << " #{@timestamp}" if @timestamp
       dump

spec/influxdb/point_value_spec.rb

@@ -6,7 +6,8 @@
       point = {
         series: '1= ,"\\1',
         tags:   {
-          '2= ,"\\2' => '3= ,"\\3'
+          '2= ,"\\2' => '3= ,"\\3',
+          '4'        => "5\\", # issue #225
         },
         values: {
           '4= ,"\\4' => '5= ,"\\5',
@@ -25,6 +26,7 @@
       series = [
         %(1=\\ \\,"\\1),
         %(2\\=\\ \\,"\\2=3\\=\\ \\,"\\3),
+        %(4=5\\ ),
       ]
       fields = [
         %(4\\=\\ \\,\\"\\4="5= ,\\"\\\\5"),