Tests fail because `m` property is missing in the MongoSnapshot

[gkubisa]

• if (meta) this.m = meta; should be this.m = meta || null; here.
• m: null property is missing here.

[alecgibson]

I raised a PR for this ages ago: #67

[ericyhwang]

Got the fix merged and published as sharedb@1.0.0-beta.6, thanks for the ping!