Pre-release #4 of Sinon 2

Changes in this release, compared to v2.0.0-pre.3

• #1061 add configurable unsafe header checks
• #1173 Expose readyState constants on XHR instances
• #1175 Fire .onprogress event handler in fake XHR
• #1176 Implement XHR.overrideMimeType
• #1183 Use last defined withArgs match when multiple conditions are met

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