Alias *field_uRI as *field_uri

When performing restrictions on a find you were required to specify the field URI keys with a capitalized "RI" which looked a bit odd and was easy to forget to do (extract taken from GenericFolder):

```ruby
{:is_greater_than_or_equal_to =>
  [{:field_uRI => {:field_uRI=>'item:DateTimeReceived'}},
    {:field_uRI_or_constant =>{:constant => {:value=>date_time.to_datetime}}}]
}}
```

This commit aliases the methods with lowercase equivalents so that the
above can be specified like this with no other changes:

```ruby
{:is_greater_than_or_equal_to =>
  [{:field_uri => {:field_uri=>'item:DateTimeReceived'}},
    {:field_uri_or_constant =>{:constant => {:value=>date_time.to_datetime}}}]
}}
```