Deleting custom header deletes all custom headers

Hi,

In the following example notice how 'custom2' header is deleted while deleting 'custom1':

```
>> m=Mail.new :custom1 => 'test1', :custom2 => 'test2'
=> \#, >

>> m[:custom2]
=> #>

>> m[:custom1] = nil
=> nil

>> m[:custom2]
=> nil
```

The problem is caused by Mail::Field#<=> method. This method treats all custom fields as equal.

Quick monkey patch:

```
class Mail::Field
def <=>( other )
self_order = FIELD_ORDER.rindex(self.name.to_s.downcase) || 100
other_order = FIELD_ORDER.rindex(other.name.to_s.downcase) || 100
if self_order == 100 && self_order == other_order
self.name.to_s.downcase <=> other.name.to_s.downcase
else
self_order <=> other_order
end
end
end
```

Start at Mail::Field#<=>, which the report identifies as treating all custom fields as equal, and reproduce the example with custom1 and custom2. Done means deleting custom1 no longer deletes custom2, while custom-field ordering remains distinct.

Written by the indexing model from the issue text.