Check how to deal with invalid values

[rfc2822]

Especially in context of #8 (comment)

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Currently, Property.parse() doesn't allow properties to fail, so something like <multistatus><getetag/></multistatus> can only be handled as an existing, but empty GetETag property (with null values in it).

However, GetETag with null values doesn't make sense.

So the following changes should be done:

• allow Property.parse() to ignore invalid exceptions (InvalidPropertyException)
• the GetETag factory throws an InvalidPropertyException when there's no ETag text
• thus the GetETag constructor can require a String (instead of String?)