Fix: Keep css attributes in logical order (#53)

[r-Larch]

This fixes the newfound problem case of #53.

This pull request will:

• Introduce a new public class CssAttributeCollection.
• Changes a Public API StyleClass.Attributes { get; set; } form Dictionary<string, CssAttribute> to CssAttributeCollection while CssAttributeCollection inherits from IDictionary<string, CssAttribute>, so there should be only a minimal breaking change.

Side effect of this change:

• Since this will keep the local order of css attributes it may also change the order of nonrelated style attributes, like: line-height and font-size as seen in PreMailer.Net/PreMailer.Net.Tests/PreMailerTests.cs

[r-Larch]

I guess this pull request could also fix #170

[martinnormark]

Very cool! Thanks for the PR, will try to cut a new release soon.

[meixger]

@martinnormark Any chance you can make a new release?

[martinnormark]

@meixger New release published: https://www.nuget.org/packages/PreMailer.Net/2.3.0

Sorry for the wait.