Can't update models with is_unique Validator

**Describe the bug**
Consider this model:
```
@kwdef mutable struct Mymodel <: AbstractModel
id::DbId = DbID()
name::String = ""
updatefield::String = ""

Validation.validator([m::Type{Mymodel}) = ModelValidator([
ValdiationRule(:name, MymodelsValidator.is_unique),
])
```
Create and save one:
```
julia > m = Mymodel(name="foo", updatefield = "bar") |> save!
```
Then try to edit it:
```
julia > m.updatefield = "baz"
julia > save!(m)
```
The validator sees the already existing Mymodel with name "foo" in the database, and doesn't let you save the updated one.

**Error stacktrace**
```
julia> save!(m)
[ Info: 2022-01-14 13:39:54 SELECT "mymodels"."id" AS "mymodels_id", "mymodels"."name" AS "mymodels_name", "mymodels"."updatefield" AS "mymodels_updatefield" FROM "mymodel" WHERE "name" = 'foo' ORDER BY mymodels.id ASC
[ Info: 2022-01-14 13:39:54 SELECT "mymodel"."id" AS "mymodels_id", "mymodels"."name" AS "mymodels_name", "mymodels"."updatefield" AS "mymodels_updatefield" FROM "mymodels" WHERE "updatefield" = 'baz' ORDER BY mymodels.id ASC
ERROR: SearchLight.Exceptions.InvalidModelException{Mymodel}(Mymodel
| KEY | VALUE |
|--------------|-------|
| updatefield::String | baz |
| id::DbId | 16 |
| name::String | foo |
, SearchLight.Validation.ValidationError[SearchLight.Validation.ValidationError(:name, :is_unique, "already exists")], "Model Mymodel is not valid: Name already exists")
Stacktrace:
[1] _save!!(m::Mymodel; conflict_strategy::Symbol, skip_validation::Bool, skip_callbacks::Vector{Symbol})
@ SearchLight ~/.julia/packages/SearchLight/B9d2o/src/SearchLight.jl:194
[2] save!!(m::Mymodel; conflict_strategy::Symbol, skip_validation::Bool, skip_callbacks::Vector{Symbol})
@ SearchLight ~/.julia/packages/SearchLight/B9d2o/src/SearchLight.jl:164
[3] #save!#40
@ ~/.julia/packages/SearchLight/B9d2o/src/SearchLight.jl:161 [inlined]
[4] save!(m::Mymodel)
@ SearchLight ~/.julia/packages/SearchLight/B9d2o/src/SearchLight.jl:161
[5] top-level scope
@ REPL[835]:1
```
If any error is thrown, please copy from the REPL and paste it here

**To reproduce**
As above

**Expected behavior**
SearchLight would recognise the object in the DB as the same as the one you are updating, and allow you to update it

**Additional context**
Please include the output of
`julia> versioninfo()`
and
`pkg> st`
```
julia> versioninfo()
Julia Version 1.6.2
Commit 1b93d53fc4 (2021-07-14 15:36 UTC)
Platform Info:
OS: Linux (x86_64-pc-linux-gnu)
CPU: Intel(R) Core(TM) i3-6100 CPU @ 3.70GHz
WORD_SIZE: 64
LIBM: libopenlibm
LLVM: libLLVM-11.0.1 (ORCJIT, skylake)
Environment:
JULIA_REVISE = auto
```
and
```
(Myproject) pkg> st
Project Myproject v0.1.0
Status `~/projects/Myproject/Project.toml`
[336ed68f] CSV v0.9.11
[c43c736e] Genie v4.9.0
[e115e502] GenieAuthentication v1.1.0
[6d011eab] Inflector v1.0.1
[e6f89c97] LoggingExtras v0.4.7
[739be429] MbedTLS v1.0.3
[0aa819cd] SQLite v1.3.0
[340e8cb6] SearchLight v2.0.1
[21a827c4] SearchLightSQLite v2.0.0
[4acbeb90] Stipple v0.19.3
[30ddb3f0] StippleCharts v0.16.0
[a3c5d34a] StippleUI v0.14.3
[ade2ca70] Dates
[56ddb016] Logging
```