Convert environment variable values to strings when posix_spawn enabled

Issue #128 pointed out that there was inconsistency between how
environment variables were treated on *nix systems by ChildProcess'
fork/exec implementation versus POSIX spawn.

Since the behavior of the more common case of fork/exec is to convert
the values to strings, we'll do the same for POSIX spawn. However, a
better long term solution would likely be to raise an error on
non-string values.

lib/childprocess/unix/posix_spawn_process.rb

@@ -111,11 +111,11 @@ def initialize(env)
           @ptrs = env.map do |key, val|
             next if val.nil?
 
-            if key =~ /=|\0/ || val.include?("\0")
-              raise InvalidEnvironmentVariable, "#{key.inspect} => #{val.inspect}"
+            if key =~ /=|\0/ || val.to_s.include?("\0")
+              raise InvalidEnvironmentVariable, "#{key.inspect} => #{val.to_s.inspect}"
             end
 
-            FFI::MemoryPointer.from_string("#{key}=#{val}")
+            FFI::MemoryPointer.from_string("#{key}=#{val.to_s}")
           end.compact
 
           @ptrs << FFI::Pointer.new(0)