Exec provider should support environment variables with empty values

The exec provider takes an environment parameter that takes either a string or an array of variables. Each variable is expressed as a pair, like "KEY=value" for example. The problem is that the code that parses this string requires a value whereas this is not strictly necessary, like "KEY=" should be perfectly acceptable. The code in Puppet::Provider::Exec.run looks like this:

if setting =~ /^(\w+)=((.|\n)+)$/

 env_name = $1

 value = $2

Simply changing the regular expression to use "*" instread of "+" for the value should work just fine:

irb(main):001:0> setting = 'KEY='

 => "KEY="

 irb(main):002:0> setting =~ /^(\w+)=((.|\n)+)$/

 => nil

 irb(main):003:0> setting =~ /^(\w+)=((.|\n)*)$/

 => 0

 irb(main):004:0> $1

 => "KEY"

 irb(main):005:0> $2

 => "" 

So, the value of empty string is exactly what was requested.