Use Space char as default delimiter

[worldbeater]

What kind of change does this PR introduce?

Now SingleLineFormatter uses the " " character by default for validation error messages concatenation. I think this behavior is what a new user would expect from the library.

What is the current behavior?

Before, it just concatenated the error message texts. This caused the messages to be glued to each other.

What is the new behavior?

Now the messages are concatenated using the whitespace character. This can be easily overriden by passing a custom SingleLineFormatter to BindValidation, but probably using the whitespace character out-of-box will make the library more developer-friendly.

What might this PR break?

Sometimes when running unit tests locally I get the following exception thrown in random tests:

System.InvalidOperationException : Operations that change non-concurrent collections must have exclusive access. A concurrent update was performed on this collection and corrupted its state. The collection's state is no longer correct.
   at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
   at ReactiveUI.POCOObservableForProperty.GetNotificationForProperty(Object sender, Expression expression, String propertyName, Boolean beforeChanged) in D:\a\1\s\src\ReactiveUI\ObservableForProperty\POCOObservableForProperty.cs:line 39
   at ReactiveUI.ReactiveNotifyPropertyChangedMixin.NotifyForProperty(Object sender, Expression expression, Boolean beforeChange) in D:\a\1\s\src\ReactiveUI\Mixins\ReactiveNotifyPropertyChangedMixin.cs:line 206
   at ReactiveUI.ReactiveNotifyPropertyChangedMixin.NestedObservedChanges(Expression expression, IObservedChange`2 sourceChange, Boolean beforeChange) in D:\a\1\s\src\ReactiveUI\Mixins\ReactiveNotifyPropertyChangedMixin.cs:line 190
   at System.Reactive.Linq.ObservableImpl.Select`2.Selector._.OnNext(TSource value) in D:\a\1\s\Rx.NET\Source\src\System.Reactive\Linq\Observable\Select.cs:line 39

This seem to happen due to a race condition in unit tests harness. The dictionary in POCOObservableForProperty is not concurrent, probably we should fix that:
https://github.com/reactiveui/ReactiveUI/blob/eb53eee4bd1865b3a226ec468c0a344620349bee/src/ReactiveUI/ObservableForProperty/POCOObservableForProperty.cs#L21

[codecov]

Codecov Report

Merging #21 into master will increase coverage by 1.69%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
+ Coverage   33.28%   34.98%   +1.69%     
==========================================
  Files          16       16              
  Lines         706      706              
  Branches       82       82              
==========================================
+ Hits          235      247      +12     
+ Misses        463      451      -12     
  Partials        8        8

Impacted Files	Coverage Δ
...iveUI.Validation/Formatters/SingleLineFormatter.cs	85.71% <100%> (ø)	⬆️
...eactiveUI.Validation/Contexts/ValidationContext.cs	84.12% <0%> (+3.17%)	⬆️
...ctiveUI.Validation/Extensions/ViewForExtensions.cs	17.64% <0%> (+5.88%)	⬆️
...Validation/ValidationBindings/ValidationBinding.cs	33.67% <0%> (+9.18%)	⬆️

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[glennawatson]

Does doing new line as the delimiter work?

[glennawatson]

Not suggesting changing that as default but I would want that as a option

[worldbeater]

Yeah, Environment.NewLine worked fine for WPF.

Not sure about other platforms btw.