Fix docs and specs for calling functions directly

When using subject.call() to test functions, the arguments need to be passed
as a single Array.

Closes #27

README.md

@@ -258,7 +258,7 @@ Puppet functions.
 
 ```ruby
 it 'should be able to do something' do
-  subject.call('foo') == 'bar'
+  subject.call(['foo']) == 'bar'
 end
 ```
 
@@ -302,7 +302,7 @@ Or by using the `call` method on the subject directly
 
 ```ruby
 it 'something' do
-  subject.call('foo', 'bar', ['baz'])
+  subject.call(['foo', 'bar', ['baz']])
 end
 ```
 
@@ -319,8 +319,8 @@ Or by using any of the existing RSpec matchers on the subject directly
 
 ```ruby
 it 'something' do
-  subject.call('foo') == 'bar'
-  subject.call('baz').should be_an Array
+  subject.call(['foo']) == 'bar'
+  subject.call(['baz']).should be_an Array
 end
 ```
 
@@ -338,7 +338,7 @@ Or by using the existing `raises_error` RSpec matcher
 
 ```ruby
 it 'something' do
-  expect { subject.call('a', 'b') }.should raise_error(Puppet::ParseError)
-  expect { subject.call('a') }.should_not raise_error(Puppet::ParseError)
+  expect { subject.call(['a', 'b']) }.should raise_error(Puppet::ParseError)
+  expect { subject.call(['a']) }.should_not raise_error(Puppet::ParseError)
 end
 ```

spec/functions/split_spec.rb

@@ -2,16 +2,11 @@
 
 describe 'split' do
   it { should run.with_params('aoeu', 'o').and_return(['a', 'eu']) }
-  it { should run.with_params('foo').and_raise_error(Puppet::ParseError) }
   it { should_not run.with_params('foo').and_raise_error(Puppet::DevError) }
 
-  it 'something' do
-    if Integer(Puppet.version.split('.').first) >= 3
-      expected_error = ArgumentError
-    else
-      expected_error = Puppet::ParseError
-    end
+  it { should run.with_params('foo').and_raise_error(Puppet::ParseError) }
 
-    expect { subject.call('foo') }.to raise_error(expected_error)
+  it 'should fail with one argument' do
+    expect { subject.call(['foo']) }.to raise_error(Puppet::ParseError)
   end
 end