fix for correctness

[simonLeary42]

These functions can't agree about how they should behave if there are pending modifications:

• attributeValueExists
  • ignores any pending modifications and use the value from $this->object
• getAttribute
  • ignores any pending modifications and use the value from $this->object
• getAttributes
  • ignores any pending modifications and use the value from $this->object
• setAttribute
  • updates the attribute in $this->mods
• setAttributes
  • discards any pending modifications and overwrites $this->mods entirely
  • setAttributes does not overwrite the entire array of attributes in $this->object, so it shouldn't do that to $this->mods
  • example where this is wrong:
    • $this->object has attributes a, b, and c
    • $this->mods has attributes c and d
    • write(); setAttributes([c => ...]); write() results in $this->object holding the d attribute
    • setAttributes([c => ...]); write() results in the d attribute being lost
• appendAttribute
  • merges $this->object, $this->mods, and the new value
  • in the case where the specified attribute is absent from $this->mods, $this->object is necessary
  • in the case where the specified attribute is present in $this->mods and is the result of another appendAttribute, the merge is technically correct but $this->object is unnecessary and confusing
  • in the case where the specified attribute is present in $this->mods and is the result of setAttribute or removeAttributeEntryByValue, this is wrong
    • example: attribute value is [$foobar], then setAttribute([]), then appendAttribute($barfoo)
      • new value should be [$barfoo]
      • actual value is [$foobar, $barfoo]
• removeAttributeEntryByValue
  • ignores $this->mods and overwrites it using the value from $this->object

With this patch, all the above functions should take pending modifications into consideration and behave the same way as though write() were called first.